We did just migrate from vCenter 5.0 to 5.1 last week but the problem started a few weeks ago when going from vCO 4.2.1 to vCO 5.1. Both are appliances so are not tied to our vCenter upgrade.
I did try the same workflow in the java client and did not see this problem. I should note that it does allow me to select the host in the webview (I am able to browse and select) but it then changes to "Not found". I'm not certain but the change seems to be related to validation running against the inputs. For example, a workflow with a 2 step presentation and host input on the first page and some other input on the second. I can select the host, move to the next step, move back to the first step, and the field will have changed to "Not found". I hope my description makes sense. 
I did open a support case with GSS but they are reluctant to look into this very far as it's technically "custom".